1. Field of the Invention
The present invention relates to moving picture data decoding, and more particularly to a selective error concealment (EC) method and apparatus for selectively concealing errors in frames upon decoding a moving picture.
2. Description of the Related Art
In general, most moving picture coding standards, which include H.263, H.264 and MPEG-4, established as international standards, employ efficient image compression schemes, such as variable length coding (VLC), spatial and motion prediction, etc. However, the variable length coding scheme has a problem in that it is very vulnerable to a noisy channel environment. For example, even a few bit errors may cause a loss of synchronism during image decoding. Also, a prediction coding scheme leads to error propagation throughout the whole image. Accordingly, errors occurring during channel transmission and storage medium recording/reading exert a negative effect on the quality of a restored image.
In order to effectively cope with such inevitable transmission errors, research has been conducted on error concealment (EC) techniques for minimizing damage due to errors occurring in a transmission environment, as well as coding schemes robust to errors.
Among a variety of EC techniques, a post-processing technique of assigning responsibility for EC to a decoder is in widespread use. The post-processing technique is a scheme for recovering error information by using such a characteristic where, in the case of a natural image, spatially or temporally adjacent pixels change smoothly. According to the post-processing technique, an erroneous block is recovered in units of pixels or macro blocks by using a pixel value or motion vector (MV) of a block neighboring the erroneous block.
FIG. 1 is a view illustrating subsequent frames output after an error occurs in a frame during decoding of a moving picture. Due to a compression scheme using correlation with a previous frame, a frame (FK+1) transmitted after a frame “FK,” in which an error occurs, is rendered in a form where the error in the previous frame is propagated. As a result, although an error occurs only in the frame “FK,” the error in the frame “FK” is propagated into frames “FK+1” and “FK+2” transmitted after the frame “FK,” so that it becomes almost impossible to identify the original images within frames “FK+1” and “FK+2.”
Moving pictures for terrestrial digital multimedia broadcasting (DMB), which has recently been activated, are implemented though a method of constructing one frame using one slice based on an H.264 compression scheme. The slice-by-slice compression method is designed to be robust to errors. According to the slice-by-slice compression method, when an error occurs within a slice, it becomes impossible to decode the entire slice. This means that error occurs frame by frame.
An error occurring in an image in which one frame is constructed using one slice is treated as an error of an entire slice, thereby causing a more serious problem in the image quality of a moving picture, as compared with the conventional pixel-by-pixel or block-by-block errors. In addition, since an erroneous region corresponds to the entire frame, it is impossible to apply the conventional block-based EC techniques.
Currently, mobile broadcasting, such as terrestrial DMB and satellite DMB, generally uses a frame freeze technique, in which, when an error exists in a moving picture of a received broadcast, an image before the error is output in the form of a still image until a new I frame is received.
FIG. 2 is a view illustrating frames output after an error occurs in a frame when a moving picture is decoded using a general frame freeze technique. As shown in FIG. 2, according to the frame freeze technique, since a frame is correctly transmitted before an error occurs in a frame is transmitted in place of the erroneous frame, it is impossible to accurately transmit image information to the user. Especially, when an error occurs in a frame, the frame freeze technique is to output a prior non-error frame on a position of the frame in which the error occurs, a user is unable to know information of the erroneous frame. In the case of general terrestrial DMB, since one I frame is received every 30 frames, when a reception error occurs, there is no choice but to discard all received video frame information until the next I frame is received, so that a viewer of mobile broadcasting has no alternative but to view a still image.